There are a number of ways of detecting smoke in a region, such as a room, building, enclosure, or open space. Some methods involve sampling air from the region and passing the sampled air through a detection chamber, whereby particles are detected and an estimation is made of the amount of smoke in the region of interest. Such an apparatus is exemplified in aspirated smoke detectors like LaserPLUS™ sold by the applicant. Other detectors are placed in the region of interest, and use a sensor to detect particles adjacent the sensor. An example of such a detector is a point detector, in which air passes between an emitter and a sensor, and the smoke is detected directly in the region of interest.
In both cases if the smoke does not enter a sampling point (of the aspirated detector) or pass between the sensor and emitter of the point detector, no smoke will be detected. As many buildings employ air handling means for extracting air from a region, such as air-conditioning, there is no guarantee that smoke will be detected rather than pass out of the region via the air handling ducts. It can be very difficult to use the aforementioned methods of detecting smoke in outdoor areas or very large indoor arenas where there may not be appropriate locations to place a point detector or a sample point and connecting tubing.
Other devices used to detect smoke include the detector disclosed in U.S. Pat. No. 3,924,252, (Duston) which uses a laser and a photodiode to detect light scattered from particles. This device uses a corner reflector to reflect the light back at the emitter. Duston requires a feedback circuit to detect whether the beam is emitted or blocked.
Another type of detector is known as a “Beam Detector”, which measures the attenuation of the intensity of a signal from a projected light source caused by smoke particles suspended in the projected light. These detectors have relatively low sensitivity and are only capable of measuring the total attenuation within the illuminated region.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.